Techniques such as infrared absorption are currently employed to monitor gas phase molecular species such as hydrogen chloride (HCl) present in samples of the atmosphere or other gases. Typically, in such systems a broad band of infrared radiation is introduced into the gas sample. Because the molecular species in the sample tend to absorb some of that radiation in characteristic wavelength bands, the resultant reduction in the intensity of the infrared output signal in these bands is measured and used to determine the presence and concentration of the species in the sample.
Conventional infrared absorption systems exhibit a number of disadvantages. The absorption bands of various species are often close to one another, or overlap, and as a result it is typically difficult to distinguish the species being monitored from those having similar absorption bands.
Also it is sometimes difficult to achieve satisfactory sensitivity using the above method. This occurs for molecular species where the average spacing of the absorption lines is significantly larger than the average width of the lines. In these instances only a small fraction of the total broad band radiation will be absorbed within the molecular absorption lines. Accurately measuring low concentrations and small changes in concentration is particularly troublesome. As a result, in order to achieve enhanced absorption and improved sensitivity present monitors have utilized fairly long path lengths and complex multiple reflection optical units. This has added to the size and expense of the detection procedure and has made many infrared systems very inconvenient to transport and use in the field. These instruments also require constant supervision by expert operators.